The present invention relates to a system for performing sequence control. More particularly, it relates to a sophisticated system available to perform complicated sequence control by employing a plurality of programmable logic controllers; each is substantially constructed much the same and is able to individually perform sequence control, nevertheless.
A sequence control system constructed having a great many mechanical relays is well known in the art and is used as a sequence controller to perform sequence control on complex machines used in manufacturing.
A programmable logic controller, or a programmable controller, however, is newly designed for its reliability, for its ease in making or changing sequence control programs performed as user programs, for its operability and for its trouble shooting capabilities if down.
Such a programmable logic controller (hereinafter, called PLC), for the most part, has limited capabilities caused by slow response speed and so on. Therefore, in case one PLC performs sequence control in accordance with its user program it is likely to limit many externally controllable devices which send input control signals to the PLC and which receive output control signals from it so as to be controlled during an execution of a sequence control program.
On the other hand, with respect to general sequence control systems, some systems include a large number of controllable devices which strain the limits of one PLC; accordingly, a large scale control system is required.
In keeping with its need for such a large scale system, it is recommended that the process ability of the PLC itself be increased a great deal. However, a PLC having a considerably increased large ability is not frequently necessary therefore, it is impossible to obtain efficiencies by mass production. Accordingly, it becomes costly to build such a sequence control system.
It is more practical to have a plurality of PLCs of medium or small capabilities parallelly operate, so as to act as an overall PLC, of the large scale type, available to control a large system, although in fact the PLCs of medium or small type, each have small ability.
As above described, a PLC has an output terminal connected to a corresponding external device to be controlled; the number of output terminals of a PLC is limitted. In case two PLCs, for example, are coupled to parallelly operate in a conventional manner, a certain output terminal of one PLC is wire connected to a certain input terminal of the other PLC; the one supplies its specific input/output state to its output terminal with respect to its program and the other receives the specific input/output state from the one PLC through its own input terminal, connected to the output terminal of the one PLC. That is, the input or output terminal connected to each other acts to exchangedly transfer signals, although in fact it is likely to be connected to an external device to be controlled. This is the same, in case two or more PLCs are coupled to parallelly operate.
Consequently, a considerable number of input and output terminals connected to external devices are used for parallelly operating a plurality of PLCs; the number of actually usable input and output terminals connected to external devices is extremely lessened. That is, an effective expansibility of a system scale to control external devices is not thusly improved; PLCs are more and more used, nevertheless.
In addition, it is necessary to correspondingly set a complicated coupling among a plurality of PLCs so as to parallelly operate them with respect to a complicated sequence program. Therefore, it is required that wiring between input and output terminals of PLCs be very complicated; it is difficult to check after completing the wiring and to alternate coupling among PLCs, once wired, so as to change sequence control performed by a parallel operation of plurality of PLCs.